1. Field of the Invention
The present invention relates to patterned metal reflow between electrical contacts on electrical interconnect substrates, and more particularly to the use of a reflowable linking metal for selective connection of electrical contacts on a customizable substrate.
2. Description of the Related Art
There is a need for making selective connections between individual electrical conductors on electrical interconnect media such as customizable high density copper/polyimide substrates. Customizable substrates have the property that some of the device interconnections are not made during the initial manufacture of the substrates, but rather are deferred. This deferral allows the substrates to be manufactured in large quantities with generic properties so that the specific design details can be applied later by the user in a final step called "customization." See, for instance, European Patent Application No. 88308996.3 (Publication No. 0310357A2) by Carey, entitled "Customizable Circuitry," which is assigned to the assignee of the present invention. Integral to such customizable substrates is a method for connecting electrical contacts in order to customize a wire routing pattern for a specific configuration of integrated circuits to be surface mounted thereon. Customizable substrates are commonly fabricated with copper conductors and a polyimide dielectric.
The use of lasers to reflow solder on a substrate or printed circuit board is well known. Numerous other solder reflow methods are available, such as vapor phase condensation, conduction, convection, hot plates, ovens, and soldering irons. Lasers, however, have distinct advantages over these other methods in solder reflow operations because the laser energy is focused into a small spot, for instance a few thousandths of an inch or smaller, without heating the surrounding regions. Laser soldering has further advantages of controlling intermetallics and finer grain structures which provide failure rates as low or lower than those of wire-wrap connections. Laser exposure energy levels and times can be accurately and repeatably controlled wherein the solder becomes molten and wets to form a reflowed solder joint.
Various laser soldering techniques include: 1) Direct reflow soldering in which tinned component leads can be reflowed to tinned pads without adding further solder by "stepping" a laser beam from joint to joint or scanning across the board if all the components are wetted with flux; and 2) Reflow soldering with solder paste in which solder paste is applied to clean untinned leads and a laser is used to melt the solder paste.
The use of a laser heating process by itself, however, has not been demonstrated to cause molten metal such as solder to wet on the polyimide surface of a copper/polyimide substrate. Instead, the molten metal will merely ball-up on the polyimide surface rather than flow across the polyimide and form a reliable electrical connection between copper conductors. Therefore, there is a need for a selective patterning process, which may employ a laser, that can make conductive connections between spaced electrical contacts on copper/polyimide substrates as well as other high density electrical interconnects.